Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g.
While such lithium-based batteries have a sufficiently high specific energy (Wh/kg) and energy density (Wh/L) to be useful in electric-powered vehicles, the practical capacity of 180 to 250 mAh/g is quite low compared to the specific capacity of lithium metal, 3861 mAh/g.
Moreover, even 250 mAh/g does not provide the necessary range for an electric/hybrid vehicle as evidenced by FIG. 1. FIG. 1 depicts a chart 10 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles.
Accordingly, to provide the desired vehicular range, a battery with a higher specific energy than the present state of the art (an intercalation system with a graphite anode and transition-metal oxide cathode) is necessary. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1794 mAh/g (based on the mass of the lithiated material), for Li2O. Other high-capacity materials include BiF3 (303 mAh/g, lithiated), FeF3 (712 mAh/g, lithiated), and others. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. Nonetheless, the theoretical specific energies are still very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes, which may enable an electric vehicle to approach a range of 300 miles or more on a single charge. Other Metal/oxygen batteries, such as Zn, Al, Si, Mg, Na, Fe, Ca, and others, also have a very high specific energy.
As noted above, batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 12 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 12, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart 12, incorporation of Li metal as the negative electrode provides a significant advantage, at least theoretically, for the lithium-oxygen system compared with the lithium-ion cells with conventional positive-electrode materials such as LiyCoO2 or LiyNi0.80Co0.15Al0.05O2. The use of other metals can also offer a higher specific energy than Li-ion cells with conventional positive electrodes.
The chart 12 further indicates that lithium/oxygen batteries, even allowing for packaging weight, are capable of providing a specific energy >600 Wh/kg and thus have the potential to enable driving ranges of electric vehicles of more than 300 miles without recharging, at a similar cost to typical lithium ion batteries. Therefore, lithium/oxygen batteries area an attractive option in the search for a battery cell that provides the desired vehicular range.
A typical lithium/oxygen electrochemical cell 20 is depicted in FIG. 3. The cell 20 includes a negative electrode 22, a positive electrode 24, a porous separator 26, and a current collector 28. The negative electrode 22 is typically metallic lithium. The positive electrode 24 includes electrode particles such as particles 30 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 32. An electrolyte solution 34 containing a salt such as LiPF6 dissolved in an organic solvent such as dimethyl ether or CH3CN permeates both the porous separator 26 and the positive electrode 24. The LiPF6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 20 to allow a high power. In some cells, the electrolyte may include LiOH or, in aqueous solutions, LiOH and LiCl.
A portion of the positive electrode 22 is enclosed by a barrier 36. The barrier 36 in FIG. 3 is configured to allow oxygen from an external source 38 to enter the positive electrode 24 while filtering undesired components such as gases and fluids. The wetting properties of the positive electrode 24 prevent the electrolyte 34 from leaking out of the positive electrode 24. Alternatively, the removal of contaminants from an external source of oxygen, and the retention of cell components such as volatile electrolyte, may be carried out separately from the individual cells. Oxygen from the external source 38 enters the positive electrode 24 through the barrier 36 while the cell 20 discharges and oxygen exits the positive electrode 24 through the barrier 36 as the cell 20 is charged. In operation, as the cell 20 discharges, oxygen and lithium ions are desired to combine to form a discharge product Li2O2 or Li2O in accordance with the following relationship:
      Li    ↔                  Li        +            +                        e          -                ⁢                                  ⁢                  (                      negative            ⁢                                                  ⁢            electrode                    )                                                  1          2                ⁢                  O          2                    +              2        ⁢                                  ⁢                  Li          +                    +              2        ⁢                                  ⁢                  e          -                      ⁢          ↔      catalyst        ⁢                  Li        2            ⁢      O      ⁢                          ⁢              (                  positive          ⁢                                          ⁢          electrode                )                                O        2            +              2        ⁢                                  ⁢                  Li          +                    +              2        ⁢                                  ⁢                  e          -                      ⁢          ↔      catalyst        ⁢                  Li        2            ⁢              O        2            ⁢                          ⁢              (                  positive          ⁢                                          ⁢          electrode                )            
The positive electrode 24 in a typical cell 20 is a lightweight, electrically conductive material which has a porosity of at least 50% to allow the formation and deposition/storage of Li2O2 in the cathode volume. The ability to deposit the Li2O2 directly determines the maximum capacity of the cell. In order to realize a battery system with a specific energy of 600 Wh/kg or greater, a plate with a thickness of 125 μm must have a capacity of about 20 mAh/cm2. Materials which provide the needed porosity include carbon black, graphite, carbon fibers, carbon nanotubes, and other non-carbon materials.
While there is a great potential for lithium-oxygen systems, there are also significant challenges that must be addressed before the lithium-oxygen system becomes commercially viable. Important challenges include reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), improving the number of cycles over which the system can be cycled reversibly, and designing a system that actually achieves a high specific energy and has an acceptable specific power.
One problem which has arisen in attempting to produce a commercially viable lithium/oxygen battery is that the practical capacity of such batteries is substantially lower than the theoretical capacity. By way of example, FIG. 4 depicts a graph 40 with three discharge curves 42, 44, and 46. The discharge curve 42 indicates the realized voltage and capacity of a metal/oxygen battery when the battery is discharged at a rate of 0.1 mA. The discharge curve 42 includes a plateau region 48 that is well below the equilibrium potential 50 of the battery, indicating a high kinetic resistance. The difference between the equilibrium potential 50 and the discharge curve 42 indicates a large difference between the actual capacity of the battery and the theoretical capacity of the battery.
The discharge curves 44 and 46 indicate the realized voltage and capacity of the metal/oxygen battery when the battery is discharged at a rate of 0.5 mA and 1.0 mA, respectively. The curves 44 and 46 indicate that at higher rates of discharge, the difference between the actual capacity of the battery and the theoretical capacity of the battery increases.
While a number of physical processes cause voltage drops within an electrochemical cell, and thereby lower energy efficiency and power output, mass-transfer limitations are an important limitation at high current densities. Precipitation of products can adversely affect mass transfer. While the formation of solid Li2O2 (or Li2O) product is desired, the Li2O2 may coat the conductive matrix of the positive electrode and/or block the pores of the electrode. By way of example, FIG. 5a depicts a carbon matrix 52 which includes a pore 54 which has a neck 56 which allows for electrolyte to communicate with the pore 54. As discharge occurs, Li and O2 combine at the surface of the carbon matrix 52 with an electron that passes through the carbon matrix 52 to form a discharge product Li2O2 58 as depicted in FIG. 5b. As the discharge product 58 forms, it may coat the entire surface of the pore 54 and neck 56 as depicted in FIG. 5c. 
The solid product 58 is thought to be electronically insulating, at least in its crystalline, bulk form. Thus, no electrons pass through the discharge product 58 and no further reduction occurs in the configuration of FIG. 5c. In some instances, the discharge product 58 closes the neck 56 as depicted in FIG. 5d. Accordingly, while electrons may be available at uncovered portions of the pore 54, no Li and O2 can pass through the neck 56 and no further reduction occurs.
Such suboptimal distribution of the Li2O2 or other solid discharge product may reduce the capacity of the cell. The transport properties of aqueous electrolytes are typically better than nonaqueous electrolytes, however, precipitation is also realized in aqueous chemistries. For example, LiOH (dissolved) and eventually LiOH.H2O (precipitated) may form in a basic aqueous medium. Consequently, in both aqueous and nonaqueous chemistries mass-transport effects may dictate cell designs with limited thickness of the various regions within the cell, including the cathode.
Unfortunately, the reduced capacity resulting from formation of solid Li2O2 (or Li2O or LiOH.H2O) product in an electrode cannot be offset simply by increasing the thickness of the electrode. In addition to weight and size considerations in applications such as electric vehicles, a number of physical processes which cause voltage drops within an electrochemical cell, and thereby lower energy efficiency and power output, are exacerbated by increased electrode thickness.
By way of example, FIG. 6 depicts a portion of a cell 60 wherein discharge product 62 has been extensively deposited on a matrix 64 at a location adjacent to a barrier 66 through which oxygen is supplied to the cell 60. The discharge product 62 adjacent to the barrier 66 thus precludes any oxygen from entering into the cell 60. Consequently, while the electrodes in the cell 60 are sufficiently thick, portions 68 of the matrix 64 are not able to support additional reactions solely because of the lack of oxygen.
Similarly, FIG. 7 depicts a cell 70 wherein discharge product 72 has been extensively deposited on a matrix 74 at a location adjacent to a separator 76. The discharge product 72 adjacent to the separator 76 thus precludes lithium (or another active material) from migrating toward a flow field/current collector 78 through which oxygen 80 is supplied to the cell 70. Consequently, while sufficient oxygen 80 is provided to the cell 70, portions of the matrix 74 adjacent to the flow field/current collector 78 are not able to support additional reactions solely because of the lack of lithium.
What is needed therefore is a metal/oxygen battery that exhibits increased capacity compared to known metal/oxygen batteries. More specifically, a metal/oxygen battery which reduces the difference between practical capacity of the battery and the theoretical capacity of the battery by controlling the uniformity of the discharge reaction product throughout thick electrodes would be beneficial.